1. Field of the Invention
The present invention relates to hydride compositions for use in storing hydrogen. In particular, the present invention relates to metal hydride compositions made so that hydrogen is absorbed as a preselected function of pressure. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Hydride formers (sometimes simply referred to as "hydrides") are capable of absorbing hydrogen which can then be desorbed under the appropriate temperature and pressure conditions. These materials have many applications, particularly in the hydrogen processing and energy conversion fields where they are used for hydrogen storage, hydrogen pumping and compression, heat pumps, batteries, and fuel cells. Many hydrides absorb the three isotopes of hydrogen (protium, deuterium, tritium) at different rates. These hydrides are useful in hydrogen isotopic purification and separation processes.
Known hydrides include many pure metals (Mg, Ti, V, Nb, Pt, Pd, and so forth), alloys (La--, Mg--, Ti--, and Co-- alloys, Ti--Fe alloys, and rare earth-Ni alloys), and hydride compositions including those described in commonly-assigned patent applications Ser. Nos. 07/933,152, filed Aug. 21, 1992 (Palladium/Kieselguhr Composition and Method), 07/952,931, filed Sep. 29, 1992 (Dimensionally Stable Metallic Hydride Composition), 07/967,653, filed Oct. 28, 1992 (Composition For Catalyzing Hydrogen Isotope Exchange), and 07/999,338, filed Dec. 31, 1992 (Tetraethyl Orthosilicate-Based Glass Composition and Method), the disclosures of which are incorporated herein by reference. The hydrogen absorption/desorption capacity of a particular hydride depends on its composition, temperature and surface area, and on the external hydrogen gas pressure. To maximize the available surface area and the absorption/desorption efficiency, hydrides are often supplied in the form of small-grained particles or pellets.
The hydrogen-storage capacity of a hydride may be determined by plotting isotherms of the hydrogen equilibrium pressure (P.sub.H.sbsb.2, P.sub.eq) versus the hydrogen content of the hydride at a predetermined temperature. At the equilibrium pressure, the partial pressure of hydrogen outside the hydride equals the pressure of the hydrogen absorbed by the hydride. The hydrogen content may be measured in any convenient units, including weight (grams H.sub.2 /grams hydride), atomic weight (mols H.sub.2 /mols hydride), or the ratio of hydrogen atoms to metal atoms in the hydride (H/M).
A series of hydrogen sorption isotherms is illustrated schematically in FIG. 1. Each isotherm 10a, 10b, 10c, 10d has a plateau region--a region of approximately constant pressure (the "plateau pressure") where the hydride absorbs or releases large quantities of hydrogen with relatively small changes in pressure. The plateau pressures for absorption and desorption may be different, a phenomenon known as hysteresis. Useful hydrides have low hysteresis, that is, absorption and desorption pressures are close and long, approximately flat plateaus.
In typical hydrides, the plateau pressure increases with temperature over the useful operating range of the material. Thus, isotherms 10a, 10b, . . . represent the hydrogen equilibrium pressure versus the hydrogen content of a single hydride, with each isotherm measured at a different temperature. The isotherms are also characteristic of the particular hydride, thus, different hydrides have isotherms with different plateau pressures and different H/M ratios. The plateaus may occur at different pressure levels and may start and end at different H/M ratios.
Hydrides with well-defined plateaus include quaternary alloys having the formula Zr.sub.1-x Ti.sub.x Cr.sub.1-y Fe.sub.1+y, where 0.05.ltoreq.x.ltoreq.0.2 and 0.ltoreq.y.ltoreq.0.4 (Lee, et al., U.S. Pat. No. 5,028,389); alloys of the formula Fe.sub.1-x Mn.sub.x Ti.sub.1-y V.sub.y, where 0.ltoreq.x.ltoreq.0.2 and 0.005.ltoreq.y.ltoreq.0.08 (Liu, et al., U.S. Pat. No. 4,358,316); and ZrMn.sub.2 -type alloys (Van Essen, et al., U.S. Pat. No. 4,489,050). Alloys that contain lanthanum and/or nickel may also have well-defined plateaus (Sasai, et al., U.S. Pat. No. 4,744,946; Sandrock, U.S. Pat. No. 4,668,424; Sandrock, et al., U.S. Pat. No. 4,409,180; Bruning, et al., 4,378,331).
For many applications, it is desirable to be able to measure the hydrogen content of a hydride with a reasonable degree of accuracy in order to determine whether the hydride is saturated or whether it can absorb more hydrogen. However, it is difficult to measure the hydrogen content directly, for example, by measuring the pressure and determining the hydrogen content from the appropriate isotherm. In the useful operating range of the hydride--the plateau region--data derived from pressure measurements are unreliable since very small pressure changes are associated with large changes in H/M. The inaccuracy in pressure measurement corresponds to a large variation in hydrogen content. Instead of using pressure as a measure of hydrogen content, hydrogen is desorbed from the hydride to a container having a known volume, and the quantity of hydrogen calculated from temperature and pressure measurements using the ideal gas law. There is a need for a way to determine hydrogen content of a hydride accurately and directly, without desorbing stored hydrogen from the composition.